Among secondary batteries, nonaqueous electrolyte secondary batteries are secondary batteries that are charged and discharged by the transfer of lithium ions between positive and negative electrodes, and since the nonaqueous electrolyte secondary batteries use an organic solvent as an liquid electrolyte, they can provide a larger voltage than those provided by nickel-cadmium secondary batteries and nickel metal hydride secondary batteries that use an aqueous solution. In nonaqueous electrolyte secondary batteries that are practically used now, lithium-containing cobalt composite oxides and lithium-containing nickel composite oxides are used as positive electrode active materials, and carbon-based materials, lithium titanate and the like are used as negative electrode active materials. Furthermore, as a liquid electrolyte, those obtained by dissolving a lithium salt such as LiPF6 and LiBF4 in an organic solvent such as cyclic carbonates and chain carbonates are used.
The positive electrode active material has an average working potential of about from 3.4 to 3.8 V versus Li/Li+, and the maximum potential during charging of from 4.1 to 4.3 V versus Li/Li+. V versus Li/Li+ is a unit of potential based on a metallic lithium potential. On the other hand, the carbon-based material and lithium titanate that are negative electrode active materials have average working potentials of about from 0.05 to 0.5 V and 1.55 V, respectively, versus Li/Li+. By combining these positive and negative electrode active materials, the battery voltage becomes from 2.2 to 3.8 V, and the maximum charge voltage becomes from 2.7 to 4.3 V.
As a countermeasure for further improving a capacity, use of LiMn1.5Ni0.5O4 having the maximum potential during charging of from 4.4 V to 5.0 V as a positive electrode active material is suggested. However, there was a problem that, when this positive electrode active material is used together with an electrolyte containing a carbonate-based solvent, the solvent causes an oxidation reaction at the positive electrode during charging, thereby the battery performance is deteriorated and generation of gas is caused. On the other hand, use of sultone- or sulfone-based compounds also as a solvent has been suggested. However, there are problems that sultone- and sulfone-based compounds have higher viscosity, provide lower solubility of lithium salts and have higher reactivity with a negative electrode as compared to carbonate-based solvents.